This invention relates to drive systems for variable speed alternating current (a-c) electric motors, and more particularly it relates to a "cycle-skipping" kind of motor speed control system.
This invention is an improvement of the cycle-skipping speed control system that is disclosed and claimed in U.S. Pat. No. 4,461,985 granted to Thomas D. Stitt on July 24, 1984, and assigned to General Electric Company, which patent is expressly incorporated herein by reference. According to the teachings of the referenced Stitt patent, a variable speed 3-phase a-c motor is connected to a 3-phase source of alternating voltage via five controllable bidirectional solid state switches each of which preferably comprises a pair of silicon controlled rectifiers or thyristors interconnected in inverse-parallel relationship with one another. The respective switches are so arranged and controlled that motor speed can be reduced by skipping selected cycles of the source voltage. For example. for half speed operation three of the switches are arranged respectively to connect phases A, B and C of the source to phases A, C and B of the motor, and over two consecutive periods of the source voltage three separate, uniformly spaced "firing windows" are selected during which the conducting states of these switches are initiated in a predetermined pattern that results in the phase A-to-phase B source voltage being applied to the motor during the first window, the phase C-to-phase A source voltage being applied during the next window, and the phase B-to-phase C source voltage being applied during the last window. Consequently the fundamental frequency of the output voltage is one-half the fundamental frequency (f) of the source voltage, and the running speed of the motor will be correspondingly reduced compared to full speed.
According to U.S. Pat. No. 4,524,316 granted jointly to Herbert J. Brown and T. D. Stitt on June 18, 1985 and assigned to General Electric Company, when the cycle-skipping system of Stitt is operating at a fraction of full speed, the magnitude-to-frequency ratio of motor voltage is maintained substantially the same as at full speed by varying the "firing angle" at which the switches start conducting current so as to minimize any deviation of the rms magnitude of the motor voltage from the product of the rms magnitude (v) of source voltage times said fraction.
The system disclosed in the prior Stitt and Brown et al patents has operated successfully in a practical application where the source frequency (f) is variable and v varies with f. Nevertheless, at reduced speed and with f in a certain below-maximum frequency range, the output current waveform of this cycle-skipping system has exhibited a relatively poor form factor and a second harmonic component of undesirably high amplitude.